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graphite films to graphene oxide without exfoliation
Journal Pre-proof Direct chemical conversion of continuous CVD graphene/graphite films to graphene oxide without exfoliation Malcolm Lockett, Viviana Sarmiento, Marquez Balingit, Mercedes Teresita OropezaGuzmán, Oscar Vázquez-Mena PII:
S0008-6223(19)31094-2
DOI:
https://doi.org/10.1016/j.carbon.2019.10.076
Reference:
CARBON 14738
To appear in:
Carbon
Received Date: 20 August 2019 Revised Date:
16 October 2019
Accepted Date: 27 October 2019
Please cite this article as: M. Lockett, V. Sarmiento, M. Balingit, M.T. Oropeza-Guzmán, O. VázquezMena, Direct chemical conversion of continuous CVD graphene/graphite films to graphene oxide without exfoliation, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.076. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Direct Chemical Conversion of Continuous CVD Graphene/Graphite films to Graphene Oxide without Exfoliation Malcolm Lockett†, Viviana Sarmiento‡, Marquez Balingit†, Mercedes Teresita Oropeza-Guzmán§ and Oscar Vázquez-Mena†. †
Department of NanoEngineering, Center for Memory and Recording Research, Cali-Baja
Center for Resilient Materials, University of California San Diego, La Jolla, CA 92093, United States. ‡
Universidad Autónoma de Baja California, Tijuana, BC. 22427, México.
§
Centro de Graduados e Investigación en Química del Instituto Tecnológico de Tijuana,
Apartado postal 1166, Tijuana, BC 22510, México.
*Corresponding Author E-mail: [email protected]
ABSTRACT
Graphene oxide (GO) is promising nanomaterial due to its surface versatility by substitution of functional groups and controllable d-spacing. However, most common methods to prepare GO are based on chemical exfoliation by strong oxidation agents forming flake dispersions that are used to form thin films of aggregated flakes. However, because of their discontinuous and amorphous morphology, such GO films have poor thermal, electronic and mechanical properties. Herein, we report a novel and quick method to obtain large and continuous GO multilayers by directly converting a chemical vapor deposited multilayer graphene (CVD-MG) into graphene oxide (CVD-MGO). Our oxidation procedure uses a CVD-MG grown on Nickel (CVD-MG/Ni), combining a gentle acid mixture with the mechanical support of the nickel foil to oxidize the CVD-MG without ripping the basal planes. XPS, EDX, and Raman spectra prove the complete conversion of MG to CVD-MGO. XRD also shows higher crystallinity for CVD-MGO than for Flake-GO film. By reducing the continuous CVD-MGO, we obtain conductive reduced-GO (CVD-rGO), which reaches 80% of the original CVD-MG conductance. Our directly conversion of CVD-MG to CVD-MGO shows great promise for improved transport properties of GO and enhanced performance for filtering and molecule separation.
*Corresponding Author E-mail: [email protected]
1. Introduction Since the experimental finding of graphene monolayers by Novosolov and Geim in 2004,[1] graphene (Gr) has been the most studied material due to its unique characteristics: its one-atom thickness nature, its record ambipolar mobility due to its massless electrons[2], stronger than steel tensile strength[3], and its facility for chemical functionalization[4]. The layered nature of graphite was discovered back in 1859 by B.C. Brodie [5], who back then observed the thin crystals that are obtained today by mechanical or chemical exfoliation. Later, in 1917, Hull developed a new X-Ray crystal analysis that allowed him to identify the hexagonal lattice of basal planes of graphite [6,7]. The extraordinary band structure of graphene was first explored theoretically by Wallace in 1947[8], which sparked a lot of interest in the experimental finding of graphite monolayers. Despite its ground-breaking characteristics, the utilization of Gr still faces several challenges including its cost-effective large-scale production, integration with current technological platforms, and expansion to novel graphene functional materials such as graphene oxide (GO) and reduced graphene oxide (rGO). Today, the best compromise between quality and quantity for synthesis of Gr is chemical vapor deposition, allowing the production of single- and multi-atomic layers of high quality Gr. Whereas the application of Gr monolayers has focused on optoelectronic applications, the development of GO films has opened broader possibilities such as water separation membranes[9–12], battery cell membranes[13,14], biosensors[15,16], and solar devices[17,18]. For these applications, the degree of oxidation and the crystalline structure are crucial for device performance[19]. GO production has been mainly carried out by chemical exfoliation, first reported by Brodie in 1859[5]. Since then, the Hummers method has become the most common method for oxidation of graphite with various alterations to the recipe[20–24]. The most popular method is the Improved Hummers Method, being the replacement of NaNO3 with H3PO4 to eliminate the dangerous gas evolution during the process [21]. These exfoliation *Corresponding Author E-mail: [email protected]
methods start with a source of bulk Gr to produce GO flakes that are then assembled out of solution into a thin film.
These films do not have the continuity of the covalent bonds,
drastically limiting many properties such as electrical, thermal conductivity and Young’s modulus[19]. The inter-flake layers also produce leaky paths that affect permeation and filtration properties. The covalent bonds can be restored through carbon lattice, but this requires hightemperature (>1000oC) annealing with graphitization conditions supplying carbon atoms with hydrocarbon sources[25–27]. Herein, we present the direct chemical conversion of large, highordered and continuous CVD multilayer graphene (CVD-MG) into multilayer graphene-oxide without disrupting the layered morphology of the original graphene layer. We called this material CVD-multilayer graphene oxide (CVD-MGO) because it keeps the layered and continuous structure of the original CVD-MG. This process does not require any layer exfoliation into loose individual flakes that results in disordered films. This improved layered structure can potentially lead to better heat, electronic and ionic transport, as well as more controlled filtration and permeability by significantly reducing vertical leaky paths present in flake films.
Our oxidation process prevents exfoliation by combining the mechanical support of the CVD Ni foil and with a gentler acidic solution for oxidation. Traditionally, Hummers method is not performed on a substrate due to the ripping of the basal planes that eventually detaches the film[28]. Figure 1.a shows the key difference between conventional Hummers methods based on flake exfoliation and our direct CVD-Hummers conversion method. To demonstrate the effective conversion of CVD-MG into CVD-MGO we use EDX, XPS and Raman analysis, showing the typical characteristics of GO reported in literature. SEM and XRD show higher crystalline morphology with respect to GO flake-films. Finally, we report the conductance measurements for our original CVD-MG, oxidized CVD-MGO, and the reduced CVD-MGO. Our results show *Corresponding Author E-mail: [email protected]
a high recovery of the conductance, with the reduced CVD-MGO having a conductance only ∼20% lower than the original CVD-MG. This highlights that our CVD-Hummers oxidation method keeps the continuity and high-quality of the basal graphene planes from CVD growth process.
Figure 1. CVD Hummers Method. (a) Conventional Hummers methods require exfoliation of graphite into flakes that are then assembled into a film, whereas our CVD-Hummers method is a direct chemical conversion without any flake exfoliation, keeping the layered structure of the original CVDMG layer. (b) CVD-Hummers method: (I) A sulphuric and phosphoric acid solution (II) is mixed with potassium permanganate. (III) Then MG/Ni is submerged into the solution and heated to 53oC. (IV) During the oxidation the solution turns purple from consumption of the reactants. (V) After 3.5 hours the CVD-MGO/Ni is removed from solution and transferred to DI water, where the CVD-MGO film is detached from the Ni foil. (VI) The CVD-MGO film is fished to the final substrate, (VII) washed in HCL and ethanol, (VIII) and finally dried out.
*Corresponding Author E-mail: [email protected]
2. Experimental 2.1 Oxidation Process Our CVD Hummers Method is described in Figure 1.b. First, (I) a mixture of 14 mL and 8 mL of 98% H2SO4 and 85% H3PO4, respectively at room temperature were poured into a beaker, then (II) 0.2g of KMnO4 was added causing the mixture to turn green due to the oxidizing agent Mn2O7/MnO3.The mixture was stirred for 5 minutes using a hand held stir bar and then CVDMG/Ni was immersed in the solution, sinking to the bottom (III). The mixture was then placed on a hot plate for 3.5 hours keeping T=53 oC, after which the solution turns brownish-purple as a result of the oxidative agent being used up in the reaction (IV)[29]. During this process, CVDMG/Ni is converted into CVD-MGO/Ni turning yellowish-brown, a clear sign of oxidation during the process[29], while remaining attached to the Ni foil at the bottom of the beaker. An optical image showing change in color is in the Supplementary Data in Figure S.1. Then, the newly formed CVD-MGO/Ni was removed from the mixture by careful handling with tweezers and washed in sequence in 100 mL DI water bath twice for 10 minutes each. In the DI water, the CVD-MGO is separated from the Ni surface (V). The delamination is shown in the Supplementary Data in Figure S.1. At this point, the film is fished with the final substrate (VI), followed by 100 mL 10% HCl bath twice for 10 minutes and ethanol bath twice for 10 minutes (VII), followed by drying with N2 gun (VIII). The total process, from immersion in the oxidative solution to final drying, is 4 Hours and 30 minutes.
2.2 Reduction Method
*Corresponding Author E-mail: [email protected]
CVD-MGO was pre-treated with hydrazine hydrate vapor for initial reduction before implementing thermal reduction in a CVD chamber. The CVD-MGO was exposed to hydrazine hydrate vapor for 2 hours at 40oC. Then, the CVD-MGO film on a Si chip was placed in the CVD chamber and was pumped down to 17 mTorr. Then 70 sccm Argon and 5 sccm Hydrogen were introduced, which increased the chamber pressure to 600 mTorr at 25 oC. The chamber was ramped to 850 oC at 17 oC min-1. The CVD-MGO film was held at 850 oC, 800 mTorr for 3 hours, then ramped down to 25 oC at 7 oC min-1.
2.3 Control Devices As control devices, Flake-Graphene Oxide (FGO) films were prepared by mixing 6.5 g of Flake-GO (2.5 wt%) with 20 mL of IPA to obtain an 8.125 mg ml-1 solution. To create samples or devices, the Flake-GO solution was spin coated onto substrate with 10 µl aliquots at 2500 rpm 4 times in sequence.
2.4 Material CVD Multilayer Graphene (500-800 nm) on Nickel (25 µm) was purchased from Graphene Supermarket. Analytical reagent grade H2SO4 (98%), H3PO4 (85%), KMnO4 and were purchased from Sigma Aldrich. HCL (10%) solution was purchased from Spectrum and Ethanol (90%) was purchased from Alfa Aesar. Highly concentrated graphene oxide flakes (2.5 wt%) was purchased from Graphenea for comparison with commercial products. The Multilayer Graphene can also be thin down by dry-etching under an O2 plasma to obtain a desired film thickness.
2.5 Characterization Methods and Tools
*Corresponding Author E-mail: [email protected]
Electron micrographs and EDX elemental analysis were obtained with a Zeiss Sigma 500 Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray (EDX) detector. The SEM images were obtained using 1-2 kV and 3 kV for CVD-MGO and CVD-MG respectively. EDX imaging was operated at 10-15 kV with a 60 or 120 µm aperture. Raman spectra were obtained using a Renishaw inVia with a 532 nm laser source, grating of 1800 grooves mm-1, and 50x objective lens. To avoid burning the samples, the power source was set to 1%. X-Ray Diffraction (XRD) data were obtained using a Rigaku Smartlab with a copper anode sealed tube source with a tungsten filament (lambda = 1.5406A) and a Nal Scintillator detector. The XRD was operated at 1.76 kW (44 kV, 40 mA) with the Parallel Beam Slit and Parallel Slit Analyzer. Sample data was collected from 5o to 55o with omega at 1o for a grazing incidence scan. X-ray Photoelectron Spectroscopy (XPS) measurements were obtained using an AXIS Supra by Kratos Analytical with an Aluminum monochromatic x-ray source (1486.6 eV) at 15 kV and emission current at 15 mA for survey and region scans. Using slot mode, the spot size is 300 x 700 µm. Water Contact Angle measurements were obtained using a Rame-Hart model 200. Surface profiles were obtained with a Dektak 150 Surface Profiler with at 10 mg force and a 12.5 µm tip. Electrical Characterization was performed using a Keithley 2400 Model with 2 probes for source and drain. Devices had pre-patterned Au electrodes for current-voltage measurements across the film.
3. Results and Discussion Confirmation of the complete interlayer conversion from CVD-MG to CVD-MGO is shown by cross-section EDX analysis. Figure 2 shows side by side SEM and EDX analysis of a CVD-MG film 700 nm thick on a SiO2/Si substrate (280 nm/525 µm thickness) and its conversion into a
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CVD-MGO film 1.6 µm thick transferred on a glass substrate (500 µm thickness) after the CVDHummers method. The SEM cross sections in Figure 2.a and 2.b show the layered structure of the films. Throughout the oxidation process CVD-MGO maintained a similar morphology to the original CVD-MG film on Ni. The preservation of morphology shows our process is not destructive towards the Gr layers nor does it create a repulsion force between the layers strong enough to exfoliate the film into flakes. Figures 2.c and 2.d show EDX Carbon signal cross section images before and after CVD-Hummers process. As expected, there are clear C signals from the graphene basal layers in both cases. In Figure 2.c. the carbon signal above the cross section originates from the top surface of the CVD-MG film. Figure 2.e and 2.f shows a clear increase in oxygen signal after the CVD-Hummer process. The thin oxygen layer and residual oxygen above it in the CVD-MG (Figure 2.e) is due to a 280 nm thick SiO2 layer under the CVD-MG. Figure 2.f shows a stronger oxygen signal in the CVD-MGO due to its oxidation. It is worth noting that EDX is performed on the cross-section that is air-exposed only after cleaving the samples, and which were not directly exposed to the oxidative agents. This shows that our CVD-Hummers method produces a film with oxidation throughout the entire film and not only on the surface and outer edges of the film. It is also worth noting that the thickness of the films can be controlled by dry-etching process. The original CVD-MG can be etched under O2 plasma, as well as the resulting CVD-MGO (Figure S.2). In terms of lateral dimensions, our samples are typically 0.5 x 0.5 cm2. Our largest produced CVD-MGO structure is 2x2 cm2 shown in Figure S.3. In our experiments, this size is limited by the size of our CVD chamber and the size of the nickel foil provided by Graphene Supermarket. Since our CVD-Hummers method relies on delamination, it is possible that lateral size affects the CVD-Hummers process, so further analysis is required to explore the limits in lateral dimensions.
*Corresponding Author E-mail: [email protected]
Figure 2. Conversion from CVD Graphene to Graphene Oxide. a) SEM cross section of CVD-MG on a SiO2/Si (280 nm/500 µm thickness) substrate. b) SEM cross section of its corresponding CVD-MGO film transferred to a glass substrate following CVD-Hummers method, showing an expansion of the film caused by the incorporation of oxygen groups between each layer while retaining a similar layered structure with respect to the CVD-MG original material. c) EDX carbon signal of CVD-MG. d) EDX Carbon signal after CVDHummer conversion, showing Gr basal plans stays in place. e) EDX weak oxygen signal of CVD-MG is very low, coming mainly from the SiO2 background layer. f) Stronger EDX oxygen signal from CVD-MGO, showing the incorporation of oxygen groups into Gr layers as result of the oxidative process.
*Corresponding Author E-mail: [email protected]
Further evidence of the direct conversion of CVD-MG into CVD-MGO was obtained by XPS and Raman analysis. XPS spectra of CVD-MG, CVD-MGO, and Flake-GO in Figures 3.a-c show the appearance of carbon-oxygen covalent bonds formed during CVD-Hummers method. As reference, our original CVD-MG has C−C sp2 (284.39eV) and C−O (285.90eV) peaks as well as a characteristic shake up satellite of Gr. After oxidation, the CVD-MGO develops two C−C peaks, sp2(~284.3eV) and sp3(~285.15eV) along with C−O(~287.21eV), C=O(~288.85eV) and O−C=O(~291eV) peaks. These peaks are also present in our control Flake-GO sample, indicating the effective conversion of CVD-MG into CVD-MGO. Figure 3.d shows the carbon bond composition for CVD-MGO and Flake-GO. They have similar percentages of C−C sp2, C=O and O−C=O bonds. However, CVD-MGO has a significantly lower percentage of sp3 C−C but higher percentage of C−O bonds. This can be attributed to Flake-GO containing a larger amount of edge groups, which are naturally sp3 C−C groups compared to our CVD-MGO having much larger continuous sheets. The higher content of epoxy C−O−C can also be attributed to more oxygen bonds in-plane than edge oxygen (C=O) bonds. The final C/O ratio of CVD-MGO and Flake-GO being 1.81 and 2.37 respectively, showing increased oxidation in our film.
Raman spectra of CVD-MG, CVD-MGO and Flake-GO provide further evidence on the conversion of CVD-MG to CVD-MGO. Figure 3.e shows a clear match for the characteristic D and G Raman peaks of GO present in CVD-MGO and Flake-GO reference. The G peak at ~1580cm-1 represents the primary vibrational mode and the D peak at ~1350cm-1 represents the breathing mode of six-atom carbon rings activated when defects are present. The Raman spectra for original CVD-MG shows the expected 2D and G peaks at 2688.13 cm-1 and 1576.48cm-1 respectively, but no D peak, confirming the CVD-MG is pristine before oxidation. Compared to
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CVD-MG, CVD-MGO exhibits broader D and G peaks and strongly reduced 2D peak. Id/Ig ratios of CVD-MGO and Flake-GO are 0.9 and 0.762 respectively, which are indicative of lattice defects related to the sp2/sp3 and oxygen bonds in Gr. CVD-MG has a negligible Id/Ig ratio due to its pristine structure. The Id/Ig from the Raman spectrum and C/O content ratio from XPS are shown in Figure 3.f, showing that our films have higher oxygen content than Flake-GO with lower basal plane tearing. Water contact angle measurements are shown in Figure S.4 (Supplementary Data), showing a change from 84o in CVD-MG to 44o in CVD-MGO, proving the conversion of hydrophobic CVD-MG to hydrophilic CVD-MGO.
*Corresponding Author E-mail: [email protected]
Figure 3. XPS and Raman of CVD-MG, CVD-MGO, and Flake-GO (FGO). Carbon XPS signal confirming covalent bonding of carbon and oxygen groups on (a) CVD-MG, (b) CVDMGO and (c) Flake-GO. (d) The comparison of carbon bond compositions between the XPS of CVD-MGO and Flake-GO show CVD-MGO has more C-O bonds (basal plane) and fewer of all other bonds (edge bonds), except C-C sp2, showing the CVD Hummer-Method has a lesser basal plane ripping effect than the Conventional Hummers Method. (e) The Raman of CVD-MG, CVD-MGO, and Flake-GO show the development of the D-peak from CVD-MG to CVD-MGO and the similarity of CVD-MGO when compared to Flake-GO though there is a decreased amount of basal plane ripping in CVD-MGO. (f) The Id/Ig ratios and C/O ratios between CVDMGO and Flake-GO confirm the increased oxygen content of CVD-MGO over Flake-GO and the increased amount of out-of-plane vibrations due to C-O bonds on CVD-MGO.
*Corresponding Author E-mail: [email protected]
As discussed earlier, one of the main motivations for this work is to achieve films with better crystalline structure than films made from flakes. Figure 4 shows SEM cross sections of CVDMGO and Flake-GO as well as XRD analysis to compare their morphology. The SEM cross section of CVD-MGO in Figure 4.a shows a more compact structure while the Flake-GO in Figure 4.b is more disorganized. More quantitative analysis is obtained by XRD, using Scherrer’s equation[30] to evaluate the crystallite size of a film, =
/(
( ))
(1)
where L is average crystallite size (nm), K is the Scherrer constant, B is the Full Width at Half Maximum (FWHM), and
is the X-ray wavelength,
is the XRD peak position. CVD-MG,
CVD-MGO, and Flake-GO have crystallite sizes of 42.64 nm, 26.88 nm, and 9.81nm respectively. CVD-MGO has a crystallite size almost 3 times larger than Flake-GO. However, we still observe a crystallite size reduction from CVD-MG to CVD-MGO due to the oxidation process. The 2θ values and interlayer spacings (D) were also extracted from XRD. For CVD-MG 2θ=26.52o and D~0.336 nm, the typical spacing of Gr layers. CVD-MGO and Flake-GO have a 2θ values of 10.56 o and 11.4o, respectively. These results in interlayer spacings of D=0.841 nm for CVD-MGO and D=0.776 nm for Flake-GO. This could be due to the difference in the oxidation process. Further evidence of the continuous nature of the CVD-MGO is shown in the Supplementary Data in Figure S.5, showing side-by-side top down SEM images of Flake-GO and CVD-MGO films. The Flake-GO clearly show the edges of the flakes/crystals ∼2 µm in lateral size in average, whereas the CVD-MGO films do not show such edges, but rather continuous films forming folds due to the transfer process.
*Corresponding Author E-mail: [email protected]
Figure 4. SEM and XRD of CVD-MGO and Flake-GO. SEM cross section of (a) CVDMGO and (b)Flake-GO. (c) The XRD of MG, CVD-MGO, and Flake-GO showing the interlayer spacing increase after the oxidation process. (d) The D-spacing and crystallite size of each film clearly indicating an expansion of the film (D) from CVD-MG to CVD-MGO, as well as a larger crystallite size (L) for CVD-MGO than Flake-GO.
*Corresponding Author E-mail: [email protected]
A major advantage of our CVD-Hummers method is keeping the continuous layered structure from the CVD process, increasing transport properties such as electrical conductivity. Electrical measurements confirm the conversion of CVD-MG to CVD-MGO, showing the decrease in conductance as the resistance increases from 5.9 Ω to >100 MΩ as shown in Figure 5.a. CVDMGO was then reduced to CVD-rMGO by thermal reduction at 850oC, recovering most of the conductance of the original CVD-MG. The resistance of CVD-rMGO drops back to 7.2 Ω, close to the resistance of the original CVD-MG of 5.9 Ω. The CVD-rMGO conductivity is 890 S/cm and of the CVD-MG film is 1689 S/cm. The conductivity of the CVD-rMGO film is higher than previous reports of graphene reduction carried out below 1000 oC.[31] Higher conductivities have been reported reaching >1000 S/cm, but requiring annealing temperatures close to 1700oC to rebuild the covalent bonds along the basal planes[25,26]. Herein, a reduction process at 850oC produced a high conductivity CVD-rMGO layer. This reflects the main capability of the presented CVD-Hummers method, allowing oxidation while preserving the continuity of the original CVD-MG layers. In addition to the conductivity recovery, the conversion of CVD-MGO into CVD-rMGO can be confirmed by the XPS spectra in Figure 5.d. XPS shows the suppression of the carbon-oxygen peaks and therefore the reduction in the oxygen content in the film. A slight nitrogen doping is also observed probably due to the exposure to hydrazine hydrate. Figure S.6 shows the comparison of the XPS spectra for CVD-MG, CVD-MGO and CVD-rMGO. Figure 5.e shows the Raman spectra of CVD-MG, CVD-MGO and CVD-rMGO films. As expected, there is a strong change in Raman after the oxidation process due to the addition of oxygen containing groups intercalating between the graphene basal layers. After the reduction, there are no drastic changes in the Raman spectra except for a small increase in the D/G peak ratio, and an extremely small recovery of the 2D peak. Similar Raman spectra of rGO films have
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been reported when reductions are carried at temperatures ∼1000 K [32]. Significant reductions in D/G ratio and increments in 2D peak require high temperatures near 2000-3000 K to reduce defects in the film. Despite our reduction process carried at 850oC, we obtain a high conductivity that we attribute to the continuous nature of the basal planes, conserved through the oxidation and reduction process of our CVD films. Conserving the continuity and integrity of the basal planes represents a major advantage in electrical conductivity of our CVD-Hummers rMGO films.
*Corresponding Author E-mail: [email protected]
NEW FIGURE
Figure 5. IV Curve of CVD-MG, CVD-MGO, and CVD-rMGO. a) Electrical measurements of CVD-MG, CVD-MGO, and CVD-rMGO with inset of measured conductivities of CVD-MG and CVD-rMGO. The current reduction from CVD-MG to CVDMGO confirms oxidation as the film becomes insulating. The CVD-rMGO recovers ∼80% of the conductance of CVD-MG, indicating the high quality of the films structure. The conductivity of the original CVD-MG is 1689 S/cm and of the reduced layer CVD-rMGO is 890 S/cm. b, c) Optical images of the CVD-MGO and its corresponding CVD-rMGO film after reduction, noted by the films change from transparent to opaque. d) XPS of CVD-rMGO showing the reduction of the film, with carbon-oxygen bonds disappearing and an increase in C-C sp2 bonds relative to C-C sp3. e) Raman spectra of CVD-MG, CVD-MGO and CVDrMGO showing the loss of the 2D peak from CVD-MG to CVD-MGO, then a slight recovery of the 2D peak as well as the change in the Id/Ig ratio from CVD-MGO to CVD-rMGO.
*Corresponding Author E-mail: [email protected]
We attribute the preservation of the film quality and integrity of the basal planes to the strong adhesion of CVD-MG to the Ni substrate and the mild oxidation conditions, keeping the CVDMG structure in place during conversion and protecting the basal planes. To keep CVD-MG on a substrate during oxidation, the adhesion force at the interface with the substrate must be large enough to prevent the intercalation of the sulfuric acid, which would otherwise detach the film as shown in the illustration of Figure S.7 in the Supplementary Data. The adhesion between CVDMG and Ni is 0.84 J/m2 leading to a d-spacing of 0.21 nm[33], smaller than that of Gr/Gr, which is less than the size of a sulphuric molecule. This allows the CVD-MG/Ni to convert to CVDMGO/Ni without the interface separating. After conversion, CVD-MGO becomes hydrophilic, allowing water to wick in between the CVD-MGO near the CVD-MGO/Ni interface, resulting in the separation of CVD-MGO from the Ni interface[34] which does not need to be exposed to further etching solutions or mechanical disturbances that may damage the film.
4. Conclusion In conclusion, we have presented an effective and direct conversion of CVD-MG into a CVDMGO. Whereas typical methods result in chemical exfoliation flakes that are later assembled into random array of flakes, our method preserves the integrity of the CVD-MG structure. EDX and XPS elemental analysis confirm the incorporation of oxygen binding groups into the film and Raman spectra show a clear match between our CVD multilayer graphene and reference flakegraphene oxide sample. XRD shows that this method produces GO films with improved crystalline structure compared to our Flake-GO sample. Electrical measurements showing the high conductivity of the reduced CVD-MGO films, close to the original CVD-MG films, indicate the layered structure and continuity of the films. A key aspect of this modified CVD*Corresponding Author E-mail: [email protected]
Hummers method is to keep the CVD-MG attached to the Ni growing substrate, enabling to keep the structure of the film during oxidation. The CVD-MGO films produced are immediately ready for use as films, opening a potential large-scale production of GO films with reusable Ni foils for CVD. We expect that the films synthesized from our CVD Hummers can have superior ionic and electronic transport for energy storage, batteries, filtering and permeability.
Appendix A. Supplementary data
Supplementary Data: Figure S.1: Optical images of CVD-MG, CVD-MGO, and the delamination effect; Figure S.2: Optical Images and Raman Spectra,of CVD-MG thinned down by dry etching then oxidized; Figure S.3: 2x2 cm CVD-MGO film; Figure S.4: Contact angle images; Figure S.5: Side-by-side SEM images of CVD-MGO and Flake-GO films; Figure S.6: XPS spectra of CVD-MG, CVD-MGO and CVD-rMGO; Figure S.7 Illustration of oxidation hinderance at the CVD-MG/Ni interface. AUTHOR INFORMATION Corresponding Author: [email protected] Author Contributions M.L. proposed and prepared the oxidized films by the modified Hummers method. V.S., M.L., and M.B. performed the characterization of the films. O.V.M. and M.T.O.G. supervised the experiments and data analysis. M.L. and O.V.M. wrote the manuscript. All authors have given approval to the final version of the manuscript.
*Corresponding Author E-mail: [email protected]
Funding Sources National Science Foundation under Award No. 1710472 ACKNOWLEDGMENT This work was supported by the National Science Foundation under Award No. 1710472. This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, NANO3, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS 1542148).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
None
S0008-6223(19)31094-2
DOI:
https://doi.org/10.1016/j.carbon.2019.10.076
Reference:
CARBON 14738
To appear in:
Carbon
Received Date: 20 August 2019 Revised Date:
16 October 2019
Accepted Date: 27 October 2019
Please cite this article as: M. Lockett, V. Sarmiento, M. Balingit, M.T. Oropeza-Guzmán, O. VázquezMena, Direct chemical conversion of continuous CVD graphene/graphite films to graphene oxide without exfoliation, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.076. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Direct Chemical Conversion of Continuous CVD Graphene/Graphite films to Graphene Oxide without Exfoliation Malcolm Lockett†, Viviana Sarmiento‡, Marquez Balingit†, Mercedes Teresita Oropeza-Guzmán§ and Oscar Vázquez-Mena†. †
Department of NanoEngineering, Center for Memory and Recording Research, Cali-Baja
Center for Resilient Materials, University of California San Diego, La Jolla, CA 92093, United States. ‡
Universidad Autónoma de Baja California, Tijuana, BC. 22427, México.
§
Centro de Graduados e Investigación en Química del Instituto Tecnológico de Tijuana,
Apartado postal 1166, Tijuana, BC 22510, México.
*Corresponding Author E-mail: [email protected]
ABSTRACT
Graphene oxide (GO) is promising nanomaterial due to its surface versatility by substitution of functional groups and controllable d-spacing. However, most common methods to prepare GO are based on chemical exfoliation by strong oxidation agents forming flake dispersions that are used to form thin films of aggregated flakes. However, because of their discontinuous and amorphous morphology, such GO films have poor thermal, electronic and mechanical properties. Herein, we report a novel and quick method to obtain large and continuous GO multilayers by directly converting a chemical vapor deposited multilayer graphene (CVD-MG) into graphene oxide (CVD-MGO). Our oxidation procedure uses a CVD-MG grown on Nickel (CVD-MG/Ni), combining a gentle acid mixture with the mechanical support of the nickel foil to oxidize the CVD-MG without ripping the basal planes. XPS, EDX, and Raman spectra prove the complete conversion of MG to CVD-MGO. XRD also shows higher crystallinity for CVD-MGO than for Flake-GO film. By reducing the continuous CVD-MGO, we obtain conductive reduced-GO (CVD-rGO), which reaches 80% of the original CVD-MG conductance. Our directly conversion of CVD-MG to CVD-MGO shows great promise for improved transport properties of GO and enhanced performance for filtering and molecule separation.
*Corresponding Author E-mail: [email protected]
1. Introduction Since the experimental finding of graphene monolayers by Novosolov and Geim in 2004,[1] graphene (Gr) has been the most studied material due to its unique characteristics: its one-atom thickness nature, its record ambipolar mobility due to its massless electrons[2], stronger than steel tensile strength[3], and its facility for chemical functionalization[4]. The layered nature of graphite was discovered back in 1859 by B.C. Brodie [5], who back then observed the thin crystals that are obtained today by mechanical or chemical exfoliation. Later, in 1917, Hull developed a new X-Ray crystal analysis that allowed him to identify the hexagonal lattice of basal planes of graphite [6,7]. The extraordinary band structure of graphene was first explored theoretically by Wallace in 1947[8], which sparked a lot of interest in the experimental finding of graphite monolayers. Despite its ground-breaking characteristics, the utilization of Gr still faces several challenges including its cost-effective large-scale production, integration with current technological platforms, and expansion to novel graphene functional materials such as graphene oxide (GO) and reduced graphene oxide (rGO). Today, the best compromise between quality and quantity for synthesis of Gr is chemical vapor deposition, allowing the production of single- and multi-atomic layers of high quality Gr. Whereas the application of Gr monolayers has focused on optoelectronic applications, the development of GO films has opened broader possibilities such as water separation membranes[9–12], battery cell membranes[13,14], biosensors[15,16], and solar devices[17,18]. For these applications, the degree of oxidation and the crystalline structure are crucial for device performance[19]. GO production has been mainly carried out by chemical exfoliation, first reported by Brodie in 1859[5]. Since then, the Hummers method has become the most common method for oxidation of graphite with various alterations to the recipe[20–24]. The most popular method is the Improved Hummers Method, being the replacement of NaNO3 with H3PO4 to eliminate the dangerous gas evolution during the process [21]. These exfoliation *Corresponding Author E-mail: [email protected]
methods start with a source of bulk Gr to produce GO flakes that are then assembled out of solution into a thin film.
These films do not have the continuity of the covalent bonds,
drastically limiting many properties such as electrical, thermal conductivity and Young’s modulus[19]. The inter-flake layers also produce leaky paths that affect permeation and filtration properties. The covalent bonds can be restored through carbon lattice, but this requires hightemperature (>1000oC) annealing with graphitization conditions supplying carbon atoms with hydrocarbon sources[25–27]. Herein, we present the direct chemical conversion of large, highordered and continuous CVD multilayer graphene (CVD-MG) into multilayer graphene-oxide without disrupting the layered morphology of the original graphene layer. We called this material CVD-multilayer graphene oxide (CVD-MGO) because it keeps the layered and continuous structure of the original CVD-MG. This process does not require any layer exfoliation into loose individual flakes that results in disordered films. This improved layered structure can potentially lead to better heat, electronic and ionic transport, as well as more controlled filtration and permeability by significantly reducing vertical leaky paths present in flake films.
Our oxidation process prevents exfoliation by combining the mechanical support of the CVD Ni foil and with a gentler acidic solution for oxidation. Traditionally, Hummers method is not performed on a substrate due to the ripping of the basal planes that eventually detaches the film[28]. Figure 1.a shows the key difference between conventional Hummers methods based on flake exfoliation and our direct CVD-Hummers conversion method. To demonstrate the effective conversion of CVD-MG into CVD-MGO we use EDX, XPS and Raman analysis, showing the typical characteristics of GO reported in literature. SEM and XRD show higher crystalline morphology with respect to GO flake-films. Finally, we report the conductance measurements for our original CVD-MG, oxidized CVD-MGO, and the reduced CVD-MGO. Our results show *Corresponding Author E-mail: [email protected]
a high recovery of the conductance, with the reduced CVD-MGO having a conductance only ∼20% lower than the original CVD-MG. This highlights that our CVD-Hummers oxidation method keeps the continuity and high-quality of the basal graphene planes from CVD growth process.
Figure 1. CVD Hummers Method. (a) Conventional Hummers methods require exfoliation of graphite into flakes that are then assembled into a film, whereas our CVD-Hummers method is a direct chemical conversion without any flake exfoliation, keeping the layered structure of the original CVDMG layer. (b) CVD-Hummers method: (I) A sulphuric and phosphoric acid solution (II) is mixed with potassium permanganate. (III) Then MG/Ni is submerged into the solution and heated to 53oC. (IV) During the oxidation the solution turns purple from consumption of the reactants. (V) After 3.5 hours the CVD-MGO/Ni is removed from solution and transferred to DI water, where the CVD-MGO film is detached from the Ni foil. (VI) The CVD-MGO film is fished to the final substrate, (VII) washed in HCL and ethanol, (VIII) and finally dried out.
*Corresponding Author E-mail: [email protected]
2. Experimental 2.1 Oxidation Process Our CVD Hummers Method is described in Figure 1.b. First, (I) a mixture of 14 mL and 8 mL of 98% H2SO4 and 85% H3PO4, respectively at room temperature were poured into a beaker, then (II) 0.2g of KMnO4 was added causing the mixture to turn green due to the oxidizing agent Mn2O7/MnO3.The mixture was stirred for 5 minutes using a hand held stir bar and then CVDMG/Ni was immersed in the solution, sinking to the bottom (III). The mixture was then placed on a hot plate for 3.5 hours keeping T=53 oC, after which the solution turns brownish-purple as a result of the oxidative agent being used up in the reaction (IV)[29]. During this process, CVDMG/Ni is converted into CVD-MGO/Ni turning yellowish-brown, a clear sign of oxidation during the process[29], while remaining attached to the Ni foil at the bottom of the beaker. An optical image showing change in color is in the Supplementary Data in Figure S.1. Then, the newly formed CVD-MGO/Ni was removed from the mixture by careful handling with tweezers and washed in sequence in 100 mL DI water bath twice for 10 minutes each. In the DI water, the CVD-MGO is separated from the Ni surface (V). The delamination is shown in the Supplementary Data in Figure S.1. At this point, the film is fished with the final substrate (VI), followed by 100 mL 10% HCl bath twice for 10 minutes and ethanol bath twice for 10 minutes (VII), followed by drying with N2 gun (VIII). The total process, from immersion in the oxidative solution to final drying, is 4 Hours and 30 minutes.
2.2 Reduction Method
*Corresponding Author E-mail: [email protected]
CVD-MGO was pre-treated with hydrazine hydrate vapor for initial reduction before implementing thermal reduction in a CVD chamber. The CVD-MGO was exposed to hydrazine hydrate vapor for 2 hours at 40oC. Then, the CVD-MGO film on a Si chip was placed in the CVD chamber and was pumped down to 17 mTorr. Then 70 sccm Argon and 5 sccm Hydrogen were introduced, which increased the chamber pressure to 600 mTorr at 25 oC. The chamber was ramped to 850 oC at 17 oC min-1. The CVD-MGO film was held at 850 oC, 800 mTorr for 3 hours, then ramped down to 25 oC at 7 oC min-1.
2.3 Control Devices As control devices, Flake-Graphene Oxide (FGO) films were prepared by mixing 6.5 g of Flake-GO (2.5 wt%) with 20 mL of IPA to obtain an 8.125 mg ml-1 solution. To create samples or devices, the Flake-GO solution was spin coated onto substrate with 10 µl aliquots at 2500 rpm 4 times in sequence.
2.4 Material CVD Multilayer Graphene (500-800 nm) on Nickel (25 µm) was purchased from Graphene Supermarket. Analytical reagent grade H2SO4 (98%), H3PO4 (85%), KMnO4 and were purchased from Sigma Aldrich. HCL (10%) solution was purchased from Spectrum and Ethanol (90%) was purchased from Alfa Aesar. Highly concentrated graphene oxide flakes (2.5 wt%) was purchased from Graphenea for comparison with commercial products. The Multilayer Graphene can also be thin down by dry-etching under an O2 plasma to obtain a desired film thickness.
2.5 Characterization Methods and Tools
*Corresponding Author E-mail: [email protected]
Electron micrographs and EDX elemental analysis were obtained with a Zeiss Sigma 500 Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray (EDX) detector. The SEM images were obtained using 1-2 kV and 3 kV for CVD-MGO and CVD-MG respectively. EDX imaging was operated at 10-15 kV with a 60 or 120 µm aperture. Raman spectra were obtained using a Renishaw inVia with a 532 nm laser source, grating of 1800 grooves mm-1, and 50x objective lens. To avoid burning the samples, the power source was set to 1%. X-Ray Diffraction (XRD) data were obtained using a Rigaku Smartlab with a copper anode sealed tube source with a tungsten filament (lambda = 1.5406A) and a Nal Scintillator detector. The XRD was operated at 1.76 kW (44 kV, 40 mA) with the Parallel Beam Slit and Parallel Slit Analyzer. Sample data was collected from 5o to 55o with omega at 1o for a grazing incidence scan. X-ray Photoelectron Spectroscopy (XPS) measurements were obtained using an AXIS Supra by Kratos Analytical with an Aluminum monochromatic x-ray source (1486.6 eV) at 15 kV and emission current at 15 mA for survey and region scans. Using slot mode, the spot size is 300 x 700 µm. Water Contact Angle measurements were obtained using a Rame-Hart model 200. Surface profiles were obtained with a Dektak 150 Surface Profiler with at 10 mg force and a 12.5 µm tip. Electrical Characterization was performed using a Keithley 2400 Model with 2 probes for source and drain. Devices had pre-patterned Au electrodes for current-voltage measurements across the film.
3. Results and Discussion Confirmation of the complete interlayer conversion from CVD-MG to CVD-MGO is shown by cross-section EDX analysis. Figure 2 shows side by side SEM and EDX analysis of a CVD-MG film 700 nm thick on a SiO2/Si substrate (280 nm/525 µm thickness) and its conversion into a
*Corresponding Author E-mail: [email protected]
CVD-MGO film 1.6 µm thick transferred on a glass substrate (500 µm thickness) after the CVDHummers method. The SEM cross sections in Figure 2.a and 2.b show the layered structure of the films. Throughout the oxidation process CVD-MGO maintained a similar morphology to the original CVD-MG film on Ni. The preservation of morphology shows our process is not destructive towards the Gr layers nor does it create a repulsion force between the layers strong enough to exfoliate the film into flakes. Figures 2.c and 2.d show EDX Carbon signal cross section images before and after CVD-Hummers process. As expected, there are clear C signals from the graphene basal layers in both cases. In Figure 2.c. the carbon signal above the cross section originates from the top surface of the CVD-MG film. Figure 2.e and 2.f shows a clear increase in oxygen signal after the CVD-Hummer process. The thin oxygen layer and residual oxygen above it in the CVD-MG (Figure 2.e) is due to a 280 nm thick SiO2 layer under the CVD-MG. Figure 2.f shows a stronger oxygen signal in the CVD-MGO due to its oxidation. It is worth noting that EDX is performed on the cross-section that is air-exposed only after cleaving the samples, and which were not directly exposed to the oxidative agents. This shows that our CVD-Hummers method produces a film with oxidation throughout the entire film and not only on the surface and outer edges of the film. It is also worth noting that the thickness of the films can be controlled by dry-etching process. The original CVD-MG can be etched under O2 plasma, as well as the resulting CVD-MGO (Figure S.2). In terms of lateral dimensions, our samples are typically 0.5 x 0.5 cm2. Our largest produced CVD-MGO structure is 2x2 cm2 shown in Figure S.3. In our experiments, this size is limited by the size of our CVD chamber and the size of the nickel foil provided by Graphene Supermarket. Since our CVD-Hummers method relies on delamination, it is possible that lateral size affects the CVD-Hummers process, so further analysis is required to explore the limits in lateral dimensions.
*Corresponding Author E-mail: [email protected]
Figure 2. Conversion from CVD Graphene to Graphene Oxide. a) SEM cross section of CVD-MG on a SiO2/Si (280 nm/500 µm thickness) substrate. b) SEM cross section of its corresponding CVD-MGO film transferred to a glass substrate following CVD-Hummers method, showing an expansion of the film caused by the incorporation of oxygen groups between each layer while retaining a similar layered structure with respect to the CVD-MG original material. c) EDX carbon signal of CVD-MG. d) EDX Carbon signal after CVDHummer conversion, showing Gr basal plans stays in place. e) EDX weak oxygen signal of CVD-MG is very low, coming mainly from the SiO2 background layer. f) Stronger EDX oxygen signal from CVD-MGO, showing the incorporation of oxygen groups into Gr layers as result of the oxidative process.
*Corresponding Author E-mail: [email protected]
Further evidence of the direct conversion of CVD-MG into CVD-MGO was obtained by XPS and Raman analysis. XPS spectra of CVD-MG, CVD-MGO, and Flake-GO in Figures 3.a-c show the appearance of carbon-oxygen covalent bonds formed during CVD-Hummers method. As reference, our original CVD-MG has C−C sp2 (284.39eV) and C−O (285.90eV) peaks as well as a characteristic shake up satellite of Gr. After oxidation, the CVD-MGO develops two C−C peaks, sp2(~284.3eV) and sp3(~285.15eV) along with C−O(~287.21eV), C=O(~288.85eV) and O−C=O(~291eV) peaks. These peaks are also present in our control Flake-GO sample, indicating the effective conversion of CVD-MG into CVD-MGO. Figure 3.d shows the carbon bond composition for CVD-MGO and Flake-GO. They have similar percentages of C−C sp2, C=O and O−C=O bonds. However, CVD-MGO has a significantly lower percentage of sp3 C−C but higher percentage of C−O bonds. This can be attributed to Flake-GO containing a larger amount of edge groups, which are naturally sp3 C−C groups compared to our CVD-MGO having much larger continuous sheets. The higher content of epoxy C−O−C can also be attributed to more oxygen bonds in-plane than edge oxygen (C=O) bonds. The final C/O ratio of CVD-MGO and Flake-GO being 1.81 and 2.37 respectively, showing increased oxidation in our film.
Raman spectra of CVD-MG, CVD-MGO and Flake-GO provide further evidence on the conversion of CVD-MG to CVD-MGO. Figure 3.e shows a clear match for the characteristic D and G Raman peaks of GO present in CVD-MGO and Flake-GO reference. The G peak at ~1580cm-1 represents the primary vibrational mode and the D peak at ~1350cm-1 represents the breathing mode of six-atom carbon rings activated when defects are present. The Raman spectra for original CVD-MG shows the expected 2D and G peaks at 2688.13 cm-1 and 1576.48cm-1 respectively, but no D peak, confirming the CVD-MG is pristine before oxidation. Compared to
*Corresponding Author E-mail: [email protected]
CVD-MG, CVD-MGO exhibits broader D and G peaks and strongly reduced 2D peak. Id/Ig ratios of CVD-MGO and Flake-GO are 0.9 and 0.762 respectively, which are indicative of lattice defects related to the sp2/sp3 and oxygen bonds in Gr. CVD-MG has a negligible Id/Ig ratio due to its pristine structure. The Id/Ig from the Raman spectrum and C/O content ratio from XPS are shown in Figure 3.f, showing that our films have higher oxygen content than Flake-GO with lower basal plane tearing. Water contact angle measurements are shown in Figure S.4 (Supplementary Data), showing a change from 84o in CVD-MG to 44o in CVD-MGO, proving the conversion of hydrophobic CVD-MG to hydrophilic CVD-MGO.
*Corresponding Author E-mail: [email protected]
Figure 3. XPS and Raman of CVD-MG, CVD-MGO, and Flake-GO (FGO). Carbon XPS signal confirming covalent bonding of carbon and oxygen groups on (a) CVD-MG, (b) CVDMGO and (c) Flake-GO. (d) The comparison of carbon bond compositions between the XPS of CVD-MGO and Flake-GO show CVD-MGO has more C-O bonds (basal plane) and fewer of all other bonds (edge bonds), except C-C sp2, showing the CVD Hummer-Method has a lesser basal plane ripping effect than the Conventional Hummers Method. (e) The Raman of CVD-MG, CVD-MGO, and Flake-GO show the development of the D-peak from CVD-MG to CVD-MGO and the similarity of CVD-MGO when compared to Flake-GO though there is a decreased amount of basal plane ripping in CVD-MGO. (f) The Id/Ig ratios and C/O ratios between CVDMGO and Flake-GO confirm the increased oxygen content of CVD-MGO over Flake-GO and the increased amount of out-of-plane vibrations due to C-O bonds on CVD-MGO.
*Corresponding Author E-mail: [email protected]
As discussed earlier, one of the main motivations for this work is to achieve films with better crystalline structure than films made from flakes. Figure 4 shows SEM cross sections of CVDMGO and Flake-GO as well as XRD analysis to compare their morphology. The SEM cross section of CVD-MGO in Figure 4.a shows a more compact structure while the Flake-GO in Figure 4.b is more disorganized. More quantitative analysis is obtained by XRD, using Scherrer’s equation[30] to evaluate the crystallite size of a film, =
/(
( ))
(1)
where L is average crystallite size (nm), K is the Scherrer constant, B is the Full Width at Half Maximum (FWHM), and
is the X-ray wavelength,
is the XRD peak position. CVD-MG,
CVD-MGO, and Flake-GO have crystallite sizes of 42.64 nm, 26.88 nm, and 9.81nm respectively. CVD-MGO has a crystallite size almost 3 times larger than Flake-GO. However, we still observe a crystallite size reduction from CVD-MG to CVD-MGO due to the oxidation process. The 2θ values and interlayer spacings (D) were also extracted from XRD. For CVD-MG 2θ=26.52o and D~0.336 nm, the typical spacing of Gr layers. CVD-MGO and Flake-GO have a 2θ values of 10.56 o and 11.4o, respectively. These results in interlayer spacings of D=0.841 nm for CVD-MGO and D=0.776 nm for Flake-GO. This could be due to the difference in the oxidation process. Further evidence of the continuous nature of the CVD-MGO is shown in the Supplementary Data in Figure S.5, showing side-by-side top down SEM images of Flake-GO and CVD-MGO films. The Flake-GO clearly show the edges of the flakes/crystals ∼2 µm in lateral size in average, whereas the CVD-MGO films do not show such edges, but rather continuous films forming folds due to the transfer process.
*Corresponding Author E-mail: [email protected]
Figure 4. SEM and XRD of CVD-MGO and Flake-GO. SEM cross section of (a) CVDMGO and (b)Flake-GO. (c) The XRD of MG, CVD-MGO, and Flake-GO showing the interlayer spacing increase after the oxidation process. (d) The D-spacing and crystallite size of each film clearly indicating an expansion of the film (D) from CVD-MG to CVD-MGO, as well as a larger crystallite size (L) for CVD-MGO than Flake-GO.
*Corresponding Author E-mail: [email protected]
A major advantage of our CVD-Hummers method is keeping the continuous layered structure from the CVD process, increasing transport properties such as electrical conductivity. Electrical measurements confirm the conversion of CVD-MG to CVD-MGO, showing the decrease in conductance as the resistance increases from 5.9 Ω to >100 MΩ as shown in Figure 5.a. CVDMGO was then reduced to CVD-rMGO by thermal reduction at 850oC, recovering most of the conductance of the original CVD-MG. The resistance of CVD-rMGO drops back to 7.2 Ω, close to the resistance of the original CVD-MG of 5.9 Ω. The CVD-rMGO conductivity is 890 S/cm and of the CVD-MG film is 1689 S/cm. The conductivity of the CVD-rMGO film is higher than previous reports of graphene reduction carried out below 1000 oC.[31] Higher conductivities have been reported reaching >1000 S/cm, but requiring annealing temperatures close to 1700oC to rebuild the covalent bonds along the basal planes[25,26]. Herein, a reduction process at 850oC produced a high conductivity CVD-rMGO layer. This reflects the main capability of the presented CVD-Hummers method, allowing oxidation while preserving the continuity of the original CVD-MG layers. In addition to the conductivity recovery, the conversion of CVD-MGO into CVD-rMGO can be confirmed by the XPS spectra in Figure 5.d. XPS shows the suppression of the carbon-oxygen peaks and therefore the reduction in the oxygen content in the film. A slight nitrogen doping is also observed probably due to the exposure to hydrazine hydrate. Figure S.6 shows the comparison of the XPS spectra for CVD-MG, CVD-MGO and CVD-rMGO. Figure 5.e shows the Raman spectra of CVD-MG, CVD-MGO and CVD-rMGO films. As expected, there is a strong change in Raman after the oxidation process due to the addition of oxygen containing groups intercalating between the graphene basal layers. After the reduction, there are no drastic changes in the Raman spectra except for a small increase in the D/G peak ratio, and an extremely small recovery of the 2D peak. Similar Raman spectra of rGO films have
*Corresponding Author E-mail: [email protected]
been reported when reductions are carried at temperatures ∼1000 K [32]. Significant reductions in D/G ratio and increments in 2D peak require high temperatures near 2000-3000 K to reduce defects in the film. Despite our reduction process carried at 850oC, we obtain a high conductivity that we attribute to the continuous nature of the basal planes, conserved through the oxidation and reduction process of our CVD films. Conserving the continuity and integrity of the basal planes represents a major advantage in electrical conductivity of our CVD-Hummers rMGO films.
*Corresponding Author E-mail: [email protected]
NEW FIGURE
Figure 5. IV Curve of CVD-MG, CVD-MGO, and CVD-rMGO. a) Electrical measurements of CVD-MG, CVD-MGO, and CVD-rMGO with inset of measured conductivities of CVD-MG and CVD-rMGO. The current reduction from CVD-MG to CVDMGO confirms oxidation as the film becomes insulating. The CVD-rMGO recovers ∼80% of the conductance of CVD-MG, indicating the high quality of the films structure. The conductivity of the original CVD-MG is 1689 S/cm and of the reduced layer CVD-rMGO is 890 S/cm. b, c) Optical images of the CVD-MGO and its corresponding CVD-rMGO film after reduction, noted by the films change from transparent to opaque. d) XPS of CVD-rMGO showing the reduction of the film, with carbon-oxygen bonds disappearing and an increase in C-C sp2 bonds relative to C-C sp3. e) Raman spectra of CVD-MG, CVD-MGO and CVDrMGO showing the loss of the 2D peak from CVD-MG to CVD-MGO, then a slight recovery of the 2D peak as well as the change in the Id/Ig ratio from CVD-MGO to CVD-rMGO.
*Corresponding Author E-mail: [email protected]
We attribute the preservation of the film quality and integrity of the basal planes to the strong adhesion of CVD-MG to the Ni substrate and the mild oxidation conditions, keeping the CVDMG structure in place during conversion and protecting the basal planes. To keep CVD-MG on a substrate during oxidation, the adhesion force at the interface with the substrate must be large enough to prevent the intercalation of the sulfuric acid, which would otherwise detach the film as shown in the illustration of Figure S.7 in the Supplementary Data. The adhesion between CVDMG and Ni is 0.84 J/m2 leading to a d-spacing of 0.21 nm[33], smaller than that of Gr/Gr, which is less than the size of a sulphuric molecule. This allows the CVD-MG/Ni to convert to CVDMGO/Ni without the interface separating. After conversion, CVD-MGO becomes hydrophilic, allowing water to wick in between the CVD-MGO near the CVD-MGO/Ni interface, resulting in the separation of CVD-MGO from the Ni interface[34] which does not need to be exposed to further etching solutions or mechanical disturbances that may damage the film.
4. Conclusion In conclusion, we have presented an effective and direct conversion of CVD-MG into a CVDMGO. Whereas typical methods result in chemical exfoliation flakes that are later assembled into random array of flakes, our method preserves the integrity of the CVD-MG structure. EDX and XPS elemental analysis confirm the incorporation of oxygen binding groups into the film and Raman spectra show a clear match between our CVD multilayer graphene and reference flakegraphene oxide sample. XRD shows that this method produces GO films with improved crystalline structure compared to our Flake-GO sample. Electrical measurements showing the high conductivity of the reduced CVD-MGO films, close to the original CVD-MG films, indicate the layered structure and continuity of the films. A key aspect of this modified CVD*Corresponding Author E-mail: [email protected]
Hummers method is to keep the CVD-MG attached to the Ni growing substrate, enabling to keep the structure of the film during oxidation. The CVD-MGO films produced are immediately ready for use as films, opening a potential large-scale production of GO films with reusable Ni foils for CVD. We expect that the films synthesized from our CVD Hummers can have superior ionic and electronic transport for energy storage, batteries, filtering and permeability.
Appendix A. Supplementary data
Supplementary Data: Figure S.1: Optical images of CVD-MG, CVD-MGO, and the delamination effect; Figure S.2: Optical Images and Raman Spectra,of CVD-MG thinned down by dry etching then oxidized; Figure S.3: 2x2 cm CVD-MGO film; Figure S.4: Contact angle images; Figure S.5: Side-by-side SEM images of CVD-MGO and Flake-GO films; Figure S.6: XPS spectra of CVD-MG, CVD-MGO and CVD-rMGO; Figure S.7 Illustration of oxidation hinderance at the CVD-MG/Ni interface. AUTHOR INFORMATION Corresponding Author: [email protected] Author Contributions M.L. proposed and prepared the oxidized films by the modified Hummers method. V.S., M.L., and M.B. performed the characterization of the films. O.V.M. and M.T.O.G. supervised the experiments and data analysis. M.L. and O.V.M. wrote the manuscript. All authors have given approval to the final version of the manuscript.
*Corresponding Author E-mail: [email protected]
Funding Sources National Science Foundation under Award No. 1710472 ACKNOWLEDGMENT This work was supported by the National Science Foundation under Award No. 1710472. This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI) of UCSD, NANO3, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS 1542148).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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